1. Field of the Invention
This invention relates to a method and apparatus for controlling molding conditions in such a manner that molded articles of uniform quality are obtained at all times irrespective of any fluctuation in the flow properties of a resin in an injection molding machine, and especially an injection molding machine which uses a mold with a hot-runner.
2. Description of the Related Art
In recent injection molding machines, the repetitive operating precision and stability of the molding machine itself have been greatly improved due to improvements in the response of the hardware constituting the control circuitry and the use of closed-loop control.
However, since the plastic material used in injection molding is produced by a complicated polymerization reaction, the occurrence of some scatter or variance in the properties of the plastic material cannot be avoided due to variances in the raw material and difficulties in controlling the polymerization reaction. In particular, in resin materials produced by the batch method, maintaining the material properties constant from one batch to another is extremely difficult.
By way of example, when the value of the MFI (melt-flow index) which is one parameter representing resin flowability and which has the greatest influence upon injection molding, is observed, in fact the actual MFI value fluctuates by approximately .+-.10% with respect to the value indicated in a catalog even if the resin is one manufactured under fairly sophisticated management and control. Furthermore, in the case of a colored material, there is of course a variance in properties from one color to another due to a difference in the pigments and a difference in the compounding of additives resulting from the difference in pigments. Even if color is the same, there is a tendency for the variance to increase due to an increase in fluctuating factors.
Even if the control precision of an injection molding machine per se is improved, a disparity in the quality of the molded articles develops because of a fluctuation in resin properties. In particular, a fluctuation in the quality (dimensions, weight, warpage, etc.) of the molded articles which arises when resin lots are changed over from one to another cannot be avoided. Accordingly, an on-site technician must monitor the molding machine at all times and sense any fluctuation in resin properties, and the technician must adjust the molding conditions so as to hold the quality of the molded articles constant. Even if the same mold is used, it is well known on site that it is always necessary to change the molding conditions when a resin of a different color is used. In any case, the state of the art is that a change in molding conditions is carried out by repetitive trial and error, which requires much time and labor.